1. Field of Invention
The present invention relates to a reflective mask blank for EUV (Extreme Ultraviolet) lithography (hereinafter referred to also as “EUV mask blank” in this specification) to be used for e.g. production of semiconductors.
Further, the present invention relates to a reflective layer-coated substrate for EUV lithography (EUVL). The reflective layer-coated substrate for EUVL is useful as a precursor for a mask blank for EUVL.
2. Discussion of Background
Heretofore, in the semiconductor industry, a photolithography method employing visible light or ultraviolet light has been used as a technique to transfer a fine pattern required to form an integrated circuit with a fine pattern on e.g. a silicon substrate. However, the conventional photolithography method has come close to its limit, while miniaturization of semiconductor devices is being accelerated. In the case of the photolithography method, the resolution limit of a pattern is about ½ of the exposure wavelength. Even if an immersion method is employed, the resolution limit is said to be about ¼ of the exposure wavelength, and even if an immersion method of ArF laser (wavelength: 193 nm) is employed, about 45 nm is presumed to be the limit. Under the circumstances, as an exposure technique for the next generation employing an exposure wavelength shorter than 45 nm, EUV lithography is expected to be prospective, which is an exposure technique employing EUV light having a wavelength further shorter than ArF laser. In this specification, EUV light is meant for light ray having a wavelength within a soft X-ray region or within a vacuum ultraviolet region, specifically for light ray having a wavelength of from about 10 to 20 nm, particularly about 13.5 nm±0.3 nm.
EUV light is likely to be absorbed by all kinds of substances, and the refractive index of substances at such a wavelength is close to 1, whereby it is not possible to use a conventional refractive optical system like photolithography employing visible light or ultraviolet light. Therefore, in EUV lithography, a reflective optical system, i.e. a reflective photomask and mirror, is employed.
A mask blank is a layered product before pattering, to be used for the production of a photomask. In the case of an EUV mask blank, it has a structure wherein a reflective layer to reflect EUV light and an absorber layer to absorb EUV light, are formed in this order on a substrate made of e.g. glass.
Usually, a protective layer is formed between the above reflective layer and the absorber layer. Such a protective layer is provided for the purpose of protecting the reflective layer, so that the reflective layer will not be damaged by an etching process to be carried out for the purpose of forming a pattern in the absorber layer.
As the reflective layer, it is common to use a Mo/Si multilayer reflective film having a molybdenum (Mo) layer as a layer to show a low refractive index to EUV light and a silicon (Si) layer as a layer to show a high refractive index to EUV light alternately stacked to have the light reflectivity improved when the layer surface is irradiated with EUV light.
For the absorber layer, a material having a high absorption coefficient to EUV light, specifically e.g. a material containing chromium (Cr) or tantalum (Ta) as the main component, is used.
Along with a trend for higher NA (numerical aperture) for an exposure device to be used for EUV lithography, an incident angle dependence of the reflectivity at the mirror surface to be used in a reflective optical system has been problematic (Patent Document 1).
Such an incident angle dependence of the reflectivity becomes problematic also in a reflective photomask and a mask blank as a precursor for such a photomask.
For example, if the specification for NA becomes 0.45 from current 0.33, the incident angle θ of EUV light will be required to satisfy the reflectivity characteristics by expanding its range to ±15° from ±11°. The index for the reflectivity characteristics to be required in such a case has not yet been specified, but at least at one wavelength within a range of from 13.2 nm to 13.8 nm, a reflectivity of EUV light of at least 50% will be required in the entire range of −15°≦θ≦+15°, and it will be required that the fluctuation range of the EUV reflectivity within the above range of θ is small. As a specific example, at least it is required that when the minimum value is represented by Rmin (%), the fluctuation range of the EUV reflectivity in the above range of θ is within Rmin× 1/10.
Further, also in a mirror to be used in a reflective optical system, a Mo/Si multilayer reflective film is used as a reflective layer, and in the case of such a Mo/Si multilayer, a deformation of the substrate caused by an internal stress based on its structure has been problematic (Patent Document 2).
And, in the case of a reflective photomask employing a Mo/Si multilayer reflective film as a reflective layer, and a mask blank to be used for such a photomask, higher flatness than a mirror to be used in a reflective optical system is required, and accordingly, a deformation of the substrate caused by an internal stress of the Mo/Si multilayer reflective film becomes more problematic.